Presently, two kinds of rare earth magnets: samarium/cobalt-based magnet, and a neodymium/iron/boron-based magnet are widely used in various fields. The neodymium, iron/boron-based magnet exhibits the highest magnetic energy product of various kinds of magnets, and the price thereof is relatively low, so that the neodymium/iron/boron-based magnet is positively adopted in various electronic equipments.
The neodymium/iron/boron-based magnet is a magnet having Nd2Fe14B crystals as a main phase, and, in some cases, the magnet is more generally referred to as “an R-T-B magnet” Herein, R is a rare earth element and/or Y (yttrium), T is mainly Fe and a transition metal represented by Ni and Co, and B is boron. An element such as C, N, Al, Si, and/or P can be substituted for part of B, so that, in this specification, at least one element selected from the group consisting of B, C, N, Al, Si, and P is denoted by “Q”, and a rare earth magnet referred to as “a neodymium/iron/boron-based magnet” is widely referred to as “an R-T-Q rare earth magnet”. In the R-T-Q rare earth magnet, R2T14Q crystal grains constitute a main phase.
Powder of a material alloy for the R-T-Q rare earth magnet is often prepared by a method including a first pulverization process in which the material alloy is coarsely pulverized, and a second pulverization process in which the material alloy is finely pulverized. For example, in the first pulverization process, the material alloy is coarsely pulverized so as to have a size of several hundreds of micrometers or less by hydrogen decrepitation process. Thereafter, in the second pulverization process, the coarsely-pulverized material alloy (coarsely-pulverized powder) is finely pulverized so as to have an average particle diameter of about several micrometers by means of a jet mill pulverization apparatus, or the like.
There are two general kinds of methods for preparing a material alloy for a magnet. The first method is an ingot casting method in which a molten alloy of predetermined composition is put into a casting mold, and is relatively slowly cooled. The second method is a rapid solidification method represented by a strip casting method, a centrifugal casting method, or the like in which a molten alloy of predetermined composition comes into contact with a single roll, a twin roll, a rotating disk, a rotating cylindrical casting mold, or the like, and is rapidly cooled, so that a solidified alloy thinner than an ingot alloy is prepared from the molten alloy.
In the case of the rapid solidification method, the cooling speed of the molten alloy is in the range of, for example, not less than 101° C./sec. nor more than 104° C./sec. The thickness of the quenched alloy prepared by the rapid solidification method is in the range of not less than 0.03 mm nor more than 10 mm. As for the molten alloy, a face thereof which is brought into contact with a cooling roll (a roll contact face) is sequentially solidified. Thus, crystals are grown into a columnar shape (a needle-like shape) from the roll contact face in the thickness direction. As a result, the rapidly solidified alloy has a fine-crystal structure including an R2T14Q crystal phase having a short axis size of not smaller than 3 μm nor larger than 10 μm and a long axis size of not smaller than 10 μm nor larger than 300 μm, and an R-rich phase (a phase in which the concentration of a rare earth element R is relatively high) which dispersedly exists in a grain boundary of the R2T14Q crystal phase. The R-rich phase is a nonmagnetic phase in which the concentration of the rare earth element R is relatively high, and the thickness thereof (corresponding to the width of the grain boundary) is 10 μm or less.
The rapidly solidified alloy is cooled in a relatively short time, so that the structure is made to be fine and a crystal grain size is small, as compared with an alloy (an ingot alloy) prepared by a conventional ingot casting method (a mold casting method). In addition, an area of the grain boundary is wide because crystal grains are finely dispersed, and the R-rich phase is superior in dispersibility because the R-rich phase is thinly spread in the grain boundary, so that the degree of sintering is improved. Therefore, in the case where an R-T-Q rare earth sintered magnet with superior properties is to be produced, the rapidly solidified alloy is used as the material.
In the case where a hydrogen gas is once occluded in a rare earth alloy (especially in a quenched alloy), and the coarse pulverization is performed by a so-called hydrogen pulverization process (in this specification, such a pulverization method is referred to as “a hydrogen decrepitation process”), an R-rich phase positioned in a grain boundary reacts with hydrogen, and expanded, so that cracks tend to occur from a portion of the R-rich phase (the grain boundary portion). Therefore, the R-rich phase frequently appears in a grain surface of powder obtained by the hydrogen pulverization of the rare earth alloy. In the case of the rapidly solidified alloy, the R-rich phase is made to be fine, and the dispersibility is high, so that the R-rich phase is especially exposed in the surface of the power obtained by hydrogen pulverization.
The above-described pulverization method by means of the hydrogen decrepitation process is disclosed in U.S. Pat. No. 6,403,024 which is incorporated in this specification.
In a known technique, in order to increase the coercive force of such an R-T-Q rare earth magnet, Dy, Tb, and/or Ho is substituted for part of rare earth element R. In this specification, at least one element selected from the group consisting of Dy, Tb, and Ho is denoted by RH.
However, the element RH added to a material alloy for an R-T-Q rare earth magnet uniformly exists not only in an R2T14Q phase as a main phase but also in a grain boundary phase, after the rapid solidification of molten alloy. The element R existing in the grain boundary phase involves a problem that the element RH does not contribute to the increase in the coercive force.
There is another problem that the existence of a lot of element RH in the grain boundary deteriorates the degree of sintering. The problem is serious when the ratio of the element RH in the material alloy is 1.5 at % or more, and the problem is remarkable in the case where the ratio is 2.0 at % or more.
A grain boundary phase portion of the rapidly solidified alloy is easily made into super fine powder (particle diameter: 1 μm or less) by the hydrogen decrepitation process and the fine pulverization process. Even if the portion is not made into fine powder, an exposed powder surface can be easily constructed. Such super fine powder may easily cause problems of oxidation and ignition, and badly affect the sintering, so that the super fine powder is removed during the pulverization process. The rare earth element exposed on the surface of a powder grain having a particle diameter of 1 μm or more is easily oxidized. In addition, the element RH is easily oxidized, as compared with Nd and Pr, so that the element RH existing in the grain boundary phase of the alloy forms a stable oxide and is not substituted for the rare earth element R as the main phase. Thus, a segregated condition is easily maintained in the grain boundary phase.
As described above, there is a problem that, in the element RH in the quenched alloy, a portion existing in the grain boundary phase is not effectively used for the purpose of improving the coercive force. The element RH is a rare element, and is expensive. For these reasons, in views of the effective use of the resources and the reduction in production cost, it is strongly required that the above-mentioned waste is avoided.
Japanese Laid-Open Patent Publication No. 61-253805 discloses a technique in which Dy is added in the form of an oxide, and the Dy is dispersed in a surface of the main phase during the sintering, so that high coercive force can be obtained with a small amount of Dy. According to the technique, however, a Dy oxide which does not contribute to the coercive force remains in the grain boundary phase, so that the use amount of Dy cannot be sufficiently reduced.
Japanese Laid-Open Patent Publication No. 3-236202 discloses a technique in which Sn is added, in addition to Dy, so that Dy existing in the grain boundary phase is concentrated into the main phase. The technique, however, involves a problem that the existence ratio of the main phase is lowered due to the existence of Sn which does not contribute to the magnetic properties, thereby lowering the saturation magnetization. In addition, the Dy remains in the grain boundary phase as an oxide, so that the effect that Dy is concentrated into the main phase is little.
A technique in which the coercive force is improved by adding Al, Cu, Cr, Ga, Nb, Mo, V, or the like without using any heavy rare earth element such as Dy, Tb, or Ho is conventionally proposed. However, the addition of any of the elements results in the generation of a phase which does not contribute to the magnetic properties, so that there exist problems such as that the saturation magnetization is lowered, or that the magnetization of the main phase is lowered.
Japanese Laid-Open Patent Publication No. 5-33076 discloses a technique in which thermal treatment at temperatures of not lower than 400° C. nor higher than 900° C. is performed for an alloy cast block, so that the aligning direction of the main phase crystals are directed to a specified orientation.
Japanese Laid-Open Patent Publication No. 8-264363 discloses a technique in which after thermal treatment at temperatures of not lower than 800° C. nor higher than 1100° C. is performed for an alloy produced by a strip casting method, grain distribution after pulverization is improved, so that the magnetic properties are improved. However, if the thermal treatment at such temperatures is performed, the fine structure which is an advantage of the strip casting method is lost, so that the coercive force is lowered in the case where the grain distribution of powder is the same. It is considered that the degree of sintering is also lowered.
Japanese Laid-Open Patent Publication No. 10-36949 discloses a technique in which, when a molten alloy is cooled by the strip casting method, the cooling speed is limited to be 1° C./min. or less in the temperature range in which the alloy temperature lowers from 800° C. to 600° C., so as to perform slow cooling. According to this method, it is described that the ratio of main phase is increased, and the residual magnetization of the sintered magnet is improved. However, the improvement in coercive force is not described.
According to the experiments of the inventors, it was found that, especially when a rapidly solidified alloy was produced by rapidly solidifying a molten alloy, much existed in the grain boundary phase. It is considered that the phenomenon occurs because the solidifying process of the molten alloy is completed before the element RH is fallen in a lattice position (site) of the rare earth element R in the main phase. Accordingly, if the hydrogen decrepitation process is performed before the rapidly solidified alloy produced by the strip casting method or the like is finely pulverized, a lot of element RH existing in the grain boundary phase is wastefully lost. Thus, there is a problem that the use efficiency of the element RH is further lowered. In addition, when the element RH included in the alloy in the grain boundary phase is increased, the degree of sintering is lowered, so that it is necessary to increase the sintering temperature.
The present invention has been conducted in view of the above-described prior-art. A main object of the present invention is to provide an R—Fe-Q rare earth magnet with effectively improved coercive force while Dy, Tb, and Ho is effectively used.
Another objective of the present invention is to provide a production method of a material alloy for an R—Fe-Q rare earth magnet, and powder thereof, and a production method of a sintering magnet using the alloy powder.